Cambridge & Great Eastern Regional Group

SCI Members' News

Chemical Christmas

An exciting Christmas lecture at Cambridge University, organised jointly by SCI and the Royal Society of Chemistry, revealed how to make a Christmas star to wish upon, generated artificial snow, and investigated ways to help Santa make his sleigh move just that little bit faster

Have you ever wondered about the science behind Christmas? On Wednesday 6 December 2006, an inquisitive gathering was treated to an enlightening explanation at the ‘Chemistry of Christmas’ lecture and demonstration, organised jointly by SCI and the Royal Society of Chemistry, at the Bristol-Myers-Squibb lecture theatre in the Department of Chemistry of the University of Cambridge

Colin Pulham and Bridgette Duncombe took their audience on a journey which illuminated and explored the science behind some familiar Christmas motifs: crackers, snow, trees, stars, Santa’s sleigh, reindeers, puddings and indigestion, all were pressed into service in this discursive, jolly, seasonal romp.

Artificial snow was conjured from polyacrylic acid, the super absorbent polymer used in nappies, and several vivid demonstrations on the theme of colour followed. Canary yellow (lead iodide), Prussian blue, fake blood, phenolphthalein pH indicator, azo dyes, and spectacular coloured flames from copper, lithium and sodium salts.

Over several minutes, a projected image of an Edinburgh cityscape was beautifully transformed from the brightness of day to dark golden twilight by the magic of the Tyndall effect. This was ingeniously achieved by interposing a large beaker of acidified thiosulphate solution which gradually became cloudy with precipitated sulphur. As the solution turned blue with the scattered light, the sky in the cityscape, in transmitted light, turned from bright day to dark night.

The team then turned their chemical knowledge to creating a star for the festive tree. Not content with going down to the shops and buying one, instead they ingeniously harnessed the photochemical reduction of a solution of iron (III) citrate and potassium ferricyanide, to produce the image of a star on a piece of filter paper. A veritable blueprint of a Christmas star.

After a brief excursion into therapeutic photochemistry using platinum complexes, they turned their attention to exploring more environmentally-friendly alternatives for propelling Santa’s sleigh (the reindeer make too much methane). What about rocket power? Several very loud bangs (and some squeaky pops from the audience) ensued as the chemistry of the hydrogen/oxygen reaction was explored. This was followed by spectacular nitrogen tri-iodide detonations, with clouds of violet iodine vapour, and a brief discussion of the use of sodium azide in the airbags of cars.

Instead of the traditional time-consuming boiling of a Christmas pudding, more rapid gratification was sought using a plate of marshmallows in a microwave oven. From the resulting semi-molten mess it was claimed that one could to derive the wavelength of a microwave (though perhaps less satisfactory culinary results). ‘Warming’ to their theme, they then demonstrated the effect of microwave energy on a bar of soap, various solvents and pieces of metal (please do not try this at home).

This was the fourth presentation made by the scientists in two days. Their hectic schedule included driving down from Edinburgh on Monday, giving two lectures at the Open University on Tuesday, followed by lectures at Dame Alice Harpur School (Bedford) and here at Cambridge on Wednesday, after which they packed up and drove back to Edinburgh. The Cambridge and Great Eastern regional group thanks them both for their energy and enthusiasm.

Thanks also go to the staff of the Department of Chemistry at Cambridge for their assistance in making this fascinating lecture and demonstration possible.

Dr Duncombe and Dr Pulham are from the School of Chemistry at Edinburgh University

John Wilkins,
Cambridge and Great Eastern Regional Group